It's quite difficult to write about female friendship without it seeming to be a very niche subject. It's a difficult balance.

This quote is saying that when someone writes about female friendships, it is difficult to make it seem like something that appeals to a wide variety of people. This is because it is a delicate situation, in which it must be written about in a way that does not come off as too “niche” or specific, yet also still accurately describe what a female friendship is really like.